1. Field of the Invention
The present invention relates to a process and installation for the adsorptive separation, in an adsorption column, of krypton from a gas mixture containing, in particular, nitrogen in addition to the krypton. The adsorption column is filled with an adsorption medium which adsorbs krypton and nitrogen over a length L extending in the direction of the inflowing gas mixture. After the adsorption of the gas components in the adsorption column, the adsorption medium is desorbed in a following process step through the intermediary of a gaseous scavenging medium. The scavenging medium flows through the adsorption column in the same direction in which the gas mixture was introduced into the adsorption column during the first process step.
Above all, it is necessary to carry out the separation of krypton from a gas mixture is from the waste gas of a reconditioning installation for spent nuclear fuel elements because of the radiation activity of the krypton isotope Kr-85, which is released during the reconditioning of the nuclear fuel. The krypton isotope Kr-85 is a .beta.-radiation emitter having half-life of 10.7 years and is contained in the krypton isotope mixture as a constituent of 7% by volume. For krypton which is present as an inert gas, only physical dissociation or separating processes are available, of which there are preferably adapted those with the adsorption of the krypton by adsorptive media, such as activated charcoal or molecular sieves, due to their assured and dependable mode of functioning for the reconditioning of nuclear fuels. The reconditioning of nuclear fuels necessitates the operation with the hot-cell technology; in effect, in chambers which are screened against the egress of radioactive emission and the operating procedures can be only carried out by remote control or through mechanical manipulators. Consequently, sought after in the technology is a simple handling of the adsorption installations.
Contained in the waste gas of a reconditioning installation, in addition to krypton and nitrogen, with the last-mentioned being a constituent of the air which is introduced into the reconditioning installation, are gas constituents such as NO.sub.x, aerosols, iodine, steam, and oxygen. As a result thereof, prior to effecting the krypton separation must be prepurged the waste gas such that, initially, there are separated out the NO.sub.x constituents, aerosols, and iodine, subsequently the remaining NO.sub.x, steam, as well as finally xenon. The remaining waste gas possesses krypton at a proportion of about 0.1% by volume, and essentially nitrogen at a proportion of 80% by volume. From this gas mixture there should be recovered krypton extensively isolated in order to reduce as much as possible the residual volume which is to be stored away. The contamination of the krypton which is to be stored away; for instance, through residual constituents of nitrogen, oxygen, xenon, should in its entirety constitute no higher than 10% by volume.
2. Discussion of the Prior Art
It is known from the disclosure of German Patent No. 22 10 264, that there can be employed an adsorption column filled with activated charcoal from the adsorptive separation of krypton, and which is charged with krypton up to the escape or discharge of krypton. The adsorbed krypton is recovered from the adsorption column through a combined vacuum and scavenging gas desorption. In this process, disadvantageous, on the one side, is the time-consuming pumping phase which is required for desorption, as well as, on the other side, the relatively low enrichment factors which are attained in the scavenging gas for krypton during the desorption of the column. A similar separating process with adsorption and desorption under temperature and pressure change is described in the specification of German Laid-Open patent application No. 26 55 936. In this process, the inert gases constituted of krypton and xenon are always commonly separated out in an adsorption column.
From the disclosure of German Laid-Open patent application 2 326 060 there is known a process for the continual separation of krypton, in which the adsorption medium travels through a cooled adsorption chamber which is streamed through by the waste gas, and thereafter a heated desorption chamber which frees the adsorbed gas constituents. The desorbed inert gases of krypton and xenon are separated in a rectisorption section. The attainable separating yield in this process depends quite extensively upon the adsorptive properties of the adsorption medium.
Another adsorption process which is described in an International patent application, which was published by the German Patent Office under No. DE 30 49 761 A1, is also quite complex. In this process, by means of sequential process stages with each having at least two adsorption columns, there are separated from the waste gas in sequence, at first the xenon, thereafter the oxygen, and finally krypton, so that pure nitrogen will then remain at the end of the adsorption steps. In order to carry out this process there are exclusively employed, over a part, specially prepared molecular sieves.
A high degree of enrichment of the krypton in the gas mixture which is to be scavenged, and a separation through the intermediary of gas chromatography is known from the disclosure of German Laid-Open patent application 32 14 825. Sought after is a further simplification in the separation or dissociation of krypton from the waste gas.